Scan mode is a mode where all scan chain flip-flops in a chip may be tested using the test methodology known in the art as boundary scan. A chip may alternately be known as an integrated circuit, integrated circuit chip, integrated circuit device, or device, which comprises a plurality of flip flops arranged in a scan chain. Scan mode may exist on all the flip-flops in an entire chip which may be configured in very long chains or which may be arranged into smaller segments. Scan mode may be utilized for factory testing. The flip-flops may be connected in a daisy chain configuration and during scan testing, designated bit patterns of logic 0's and logic 1s may be shifted through the daisy-chained flip-flops. A control circuit may be utilized to clock data that has been shifted into the daisy-chained flip-flops in order to shift the clocked data by one flip-flop. The resulting shifted data may then be shifted back out and compared with the data that was shifted in order to determine whether there are any stuck bits, broken links, open circuits, shorts, and/or other defects.
In certain instances, important information such as security keys may be stored in the flip-flops of a chip when the chip is functioning in normal operating mode. Whenever this happens, all the data in the flip-flop may be shifted out of the scan chain in order to capture all of the states of the flip-flops. These states of the flip-flops may include information pertaining to all of the keys. Accordingly, with knowledge of the internals of the chip, one skilled in the art may be able to assemble all this information and decipher the keys in order to compromise data integrity.
In set-top boxes that have conditional access systems, for example, a hacker may halt the normal operation of the chip and enter scan mode, which is utilized to test the flip-flops. Once access is gained to the flip-flop during scan mode, the data in the flip-flops may be acquired by shifting out the contents of at least a portion of the flip-flops that have secure information stored therein. Accordingly, access to scan mode should be prevented during operation to ensure integrity of data such as security keys that are stored in the flip-flops of a chip.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.